Intel Processor Power Delivery Guidelines

If you've ever overclocked a system, chances are that at some point or another you've had opportunity to become upset with your Vdroop "problem." Some users, confused as to why their system refuses to exactly match actual processor supply voltage to the value specified in BIOS, are quick to blame the quality their motherboard; still others find fault with the difference noted between their board's idle and full-load processor supply voltages. Actually, load line droop (Vdroop) is an inherent part of any Intel power delivery design specification and serves an important role in maintaining system stability. In most cases, comments regarding unacceptable power delivery performance are completely unfounded. To make matters worse, unjustified negative consumer perception surrounding this often misunderstood design feature eventually forced a few motherboard manufacturers to respond to enthusiasts' demands for action by adding an option in their BIOS that effectively disables this important function.

Based on the currently running tasks, processor load can vary significantly during system operation. The voltage regulator module (VRM) circuit closely regulates CPU supply voltage by sensing instantaneous changes in processor loading and then responds by varying the individual on-time for a bank of power MOSFETs used to charge a multi-phased LC network. This LC network is responsible for providing all of the power demanded by the processor. If the VRM senses a decreasing supply voltage, it provides more current; the opposite is true in the case where voltage is rising. This cycle of sense-and-correct, known as negative feedback, can occur at that rate of thousands to millions of times per second, depending on the particular circuit's switching frequency.


Motherboard
VRM Supply Current
Just like CPU power, CPU supply current increases quickly at higher frequencies

During periods of high CPU demand, the VRM circuit works hard to supply the current required by the processor. However, as soon as that load is gone, the VRM circuit must act quickly in order to reduce the current supply to the level needed to match the new demand. Because it's impossible for the VRM circuit to respond instantaneously, the larger the load change the greater the maximum potential peak overshoot voltage. Controlling the magnitude of these peak values is critical for maintaining system stability. By positioning the processor's no-load (idle) voltage level higher during periods of light loading, it's possible to sustain a larger negative voltage dip without crossing the processor's lower specified voltage limit. In addition, "drooping" the load voltage as a function of supply current allows the VRM to effectively limit the maximum positive peak overshoot voltage (experienced during a heavy to light load transient) to a value below the maximum allowable CPU voltage. This resulting control system ensures the processor supply voltage, regardless of CPU load, never violates a specified limit. The following figure should help to illustrate these concepts.



As intended, Voffset and Vdroop ensure that the supply voltage never exceeds CPU VID

The CPU VID setting establishes the absolute maximum allowable processor supply voltage experienced during transient conditions and is not the target idle voltage. We hope this statement draws attention to this important distinction, as many believe the opposite to be true - a mistake all too commonly made. Together, Vdroop and Voffset ensure that the peak CPU supply voltage seen during heavy to light loading changes remains well below the established maximum. If you determine that 1.17V, as in the case above, is not sufficient for maintaining CPU stability under load, simply increasing the CPU VID does correct the problem. Let's now examine how the system responds if we remove Voffset.



Voltage oscillations while leaving heavy load can cause problems with no Voffset

As we can see, the system exceeds maximum allowable processor voltage whenever any heavy to light load transient is significant enough to warrant one or more voltage excursions above the CPU VID value. Even worse, this all happens without the user's knowledge. Again, removing Voffset completely undermines the purpose of the VID setting - which establishes the maximum CPU voltage, not the target value.

An Unexpected Loss of Performance at Higher Speeds Intel Processor Power Delivery Guidelines (Cont'd)
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  • Griswold - Thursday, December 20, 2007 - link

    I think the other explanation for the delay is much more likely than this horsecrap. Reply
  • tomoyo - Wednesday, December 19, 2007 - link

    This is a great article from both a readability and technical standpoint, I found it very enjoyable to actually read the commentary, which I cannot say is true for 95% of tech reviews. Also the graphs and information presented were extremely useful. I love the demystifying of the vdroop issue to prove that it's a GOOD thing. Reply
  • Amiteriver - Wednesday, December 19, 2007 - link

    Finally a review with some understandable explanations of Bios settings and there realation ships. As the motherboards change you get new Bios names for things and your eyes glaze over wondering what the heck some of the terms mean. Look forward to future articles of popular motherboards Bioses. I will be printing this article out. Thanks much Reply
  • Amiteriver - Wednesday, December 19, 2007 - link

    Finally a review with some understandable explanations of Bios settings and there realation ships. As the motherboards change you get new Bios names for things and your eyes glaze over wondering what the heck some of the terms mean. Look forward to future articles of popular motherboards Bioses. I will be printing this article out. Thanks much Reply
  • Sczee - Wednesday, December 19, 2007 - link

    Quite possibly the best article on computer hardware I have ever read. I learnt a lot about overclocking from it and plan to put it to use tonight. I'll be looking out for your next piece Kris. Reply
  • Bozo Galora - Wednesday, December 19, 2007 - link

    ya, the X48 is being delayed because some top tier mobo manuf needs to get rid of its X38 inventory. My guess is Asus, because they have the muscle to influence Intel. Bleh.

    So I bought all this super xmas bargain stuff, and now I am going to have to let it gather dust - lol

    http://www.digitimes.com/mobos/a20071217PD205.html">http://www.digitimes.com/mobos/a20071217PD205.html
    Reply
  • Bozo Galora - Wednesday, December 19, 2007 - link

    and the X48 does seem to have a 450-460 cap, at least so far

    http://en.hardspell.com/doc/showcont.asp?news_id=1...">http://en.hardspell.com/doc/showcont.asp?news_id=1...
    Reply
  • bryanW1995 - Wednesday, December 19, 2007 - link

    Best article I've read here in a long time. Good job. Reply
  • Regs - Wednesday, December 19, 2007 - link

    So lets say I have a 65nm Core Duo running between 0.850V-1.3525V. These are the product specs, which I guess .850 is the low limit and 1.325 is the high limit. Why does the voltage have to decrease depending on load? Is it just as simple as "supply and demand"? How does running the CPU at 1.352v run the risk of instability? Reply
  • Aivas47a - Wednesday, December 19, 2007 - link

    Great article. You guys have really been distinguishing yourselves with in-depth work on overclocking the last few months: exploring obscure bios settings, tinkering with "extreme" cooling -- keep it up!

    My experience with a qx9650 so far is very similar to yours: easy scaling to 4 ghz, difficult scaling after that with 4.2 ghz being the practical max for regular operation (folding, etc.).

    One issue I will be interested to see you address in the future is fsb overclocking on yorkfield. So far I am seeing yorkfield top out at lower fsb (450-460) than was possible for kentsfield on a comparable P35 or X38 platform. That is not so significant for the unlocked Extreme Edition chips, but could make it difficult to achieve the magic 4 ghz with the q9550 and especially the q9450.
    Reply

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